Helically wound pipes of the foregoing type are often used for carrying pressurized fluids. In particular, they find application in pneumatic devices generally useful in industry. In particular, they are found on trucks and tractor-trailer vehicles and act as a flexible connection between the towing vehicle and the trailer. Because of their ability to expand and contract, they obviate the necessity for reels, counter-weights, etc. for rolling up or stowing the required length of pipe.
German Patent 31 38 021 describes the process of manufacturing an untwisted wire helix, whose deformation forces are compensated after setting by an additional coaxial twist contracting the helix. Regarding German Patent 29 40 052, the twist stress resulting from the deformation manufacturing process forming the helix is reduced after having been set by a coaxial but opposite rotation. It is doubtful that pipes having larger wall thickness from polymers can be produced using these procedures.
In the past, such pipes have been produced by winding and thermofixing. U.S. Pat. Nos. 3,021,871 and 3,245,431, as well as British Patent 1,518,424 teach processes of this kind. However, the ability of such helical pipes to return to their original length on release of tension is limited and unsatisfactory.
In an effort to overcome this problem, multi-layer construction has been used. Also, suitable resilience has been sought by way of blending the polymer components of the material of which the pipe is made. However, with such modifications, it cannot be used in pipes which are intended for safety components and compressed air installations. This is particularly true if they are to come into contact with materials which have a solubilizing or extracting effect thereon. Such substances as fuels, lubricating oils, hydraulic oils, lacquers, and solvents all fall into this category. Furthermore, when the pipes are subjected to UV radiation or heat, embrittlement can occur, thereby rendering the pipes useless and even dangerous.
The measurement of the resilience of such pipes, particularly those intended for use in connection with air brakes, is set forth in DIN 74323 and in SAE J 844. In accordance with the former, for example, the helix is expanded for 60 seconds to a length of 5 meters by a maximum force of 50 N and the length l.sub.s is measured 5 minutes after release. In accordance with the following equation, the restoring factor R is calculated. EQU R=(l.sub.s -a-b)/(l.sub.o -a-b)
wherein l.sub.o is the original length of the pipe before the application of tension and a and b are the lengths of the connectors from each fitting to the beginning of each helix.